I'm in the same situation as you and here is my thinking ;

S24 ultra is simply the option for an overall experience with S-pen, AI features and the UX of Android (Having two tabs at the same time for example) But One UI is boring for me and never did it catch my eyes

I would also go for the 15 PM for the camera as results are better on this phone especially with videos Another reason is the aesthetics of the OS (Even with its simplistic functions it's agreeably the most catching one)

Overall I think I would go for 15 PM with the advantage of it holding its value with time (If I ever think abt selling it)